fix JiveXMLServer mutex lock and corrupted ERS error message

JiveXMLServer tries to copy the event stream identifier and event string after the mutex lock is released, which is not thread safe. This has caused occasional crashes of the JiveXML-Server application in the online event displays, and is linked to the issue of incomplete events being sent to Atlantis under input high rate. This MR changed the order and only release the mutex lock after the copy is done.

This MR also fixed the corrupted error message from JiveXMLServer: In the GNU-specific strerror_r(), buf may or may not be filled but the return value always points to the error message, so we use the return value instead of buf.